This invention relates to a near-field optical probe for observing, measuring and forming optical characteristics in a microscopic region of a sample, and a manufacturing method for the same.
At present, in the scanning near-field microscope (hereinafter, abbreviated as SNOM), an optical medium having a microscopic aperture in a sharpened tip is used as a probe. The tip and microscopic aperture is approached to a measured sample to a distance of less than a wavelength of light to measure an optical characteristic and shape of the sample with resolution. In this apparatus, a linear optical fiber probe vertically held to a sample at a tip is horizontally vibrated with respect to a sample surface. The detection of change in vibration amplitude caused by a shearing force acting between the sample surface and the probe tip is made by illuminating laser light to the probe tip and detecting a change of a shade thereof. The distance between the probe tip and the sample surface is held constant by moving the sample by a fine movement mechanism in a manner of making amplitude constant. From a signal intensity inputted to the fine movement mechanism, a surface shape is detected and a sample optical characteristic is measured. Such apparatus is proposed.
A scanning-type near-field atomic microscope has also been proposed which uses an optical fiber probe formed in a hook form as a cantilever for an atomic force microscope (hereinafter, abbreviated as AFM) to illuminate laser light to a sample from a tip of the optical fiber probe simultaneous with AFM operation to detect a surface shape and measure a sample optical characteristic (Japanese Patent Laid-open No. 174542/1995). FIG. 16 is a structural view showing a conventional optical fiber probe. This optical fiber probe uses an optical fiber 501 covered at a periphery by a metal film coating 502. A probe needle portion 503 is sharpened and has an aperture 504 at a tip of the probe needle portion 503.
On the other hand, in the AFM utilized as shape observing means for microscopic regions, a silicon or silicon-nitride micro-cantilever manufactured by a silicon process is broadly utilized. The micro-cantilever used in the AFM has a feature in mechanical characteristic such as in spring constant and resonant frequency because of high resonant frequency, good mass producibility and less variation in shape. By forming a microscopic aperture in a tip of the micro-cantilever used in AFM, as shown in FIG. 17, a probe for SNOM is known which is formed by a tip 505, a lever 506, a base 507, microscopic aperture 508 and a shade film 509 (S. Munster et al., Novel micromachined cantilever sensors for scanning near-field optical microscopy, Journal of Microscopy, vol. 186, pp17-22, 1997). Here, a tip 505 and a lever 506 are formed of silicon nitride or silicon. By incidence of light on the SNOM probe as shown at Light in FIG. 17, near-field light can be illuminated from the microscopic aperture 508.
However, the optical fiber probe shown in FIG. 16 is poor in mass producibility because of manual manufacture one by one. Also, because the optical fiber 501 is used as a light-propagating member, the difference in propagation characteristic by wavelength is great and difficult in use for spectroscopic analysis.
Although the SNOM probe shown in FIG. 17 is easy to mass-produce by a silicon process, foreign matter including dust in air readily intrudes into a recess in a tip portion. Accordingly, there has been a problem that near-field light illuminated from the microscopic aperture is not stabilized in intensity. Further, where the tip in position is formed at a tip of the cantilever, a spot of incident light is off the cantilever during introduction of light into the microscopic aperture. When detecting an optical signal from a sample by the microscopic aperture, optical signals at other than the tip end are detected. consequently, there has been a problem that the optical image of SNOM is worsened in optical-image S/N ration. Further, because the tip is formed using mold formed of anisotropic etching of silicon, the tip at an end angle is fixed as 70 degrees. Accordingly, there has been a problem that the near-field light illuminated from the microscopic aperture cannot be increased in intensity. Further, the lever 506 and the tip 505 are structured of a material small in reflectivity relative to a wavelength of incident light or the light detected by the microscopic aperture. In the NOM probe shown in FIG. 17, because the structural material of them is in an optical path, the intensity of incident light or detection light attenuates due to reflection upon the structural material. There has been a problem that the near-field light illuminated from the microscopic aperture 508 and the light detected by the microscopic aperture 508 are decreased in intensity.
Therefore, this invention has been made in view of the above, and it is an object to provide a near-field optical probe having a cantilever for SNOM to illuminate and/or detect light through a microscopic aperture, which is excellent in mass-producibility and evenness, capable of obtaining an intensity of stable near-field light without intrusion of foreign matter to a tip portion, improves optical-image S/N ratio by shading leak light and capable of obtaining great near-field light intensity, and a method for manufacturing same.
Therefore, the present invention has a structure, in a near-field optical probe for observing and measuring optical information in a microscopic region of a sample by generating and/or detecting near-field light, comprising: a cantilever; a base for supporting the cantilever; a tip in the form of a conical or pyramidal formed on the cantilever in a surface opposite to a surface of the base; a microscopic aperture formed in an end of the tip; a shade film formed on the surface of the cantilever opposite to the surface of the base and on a surface of the tip excepting the microscopic aperture; wherein the tip and the cantilever are formed using a transparent material high in transmissivity for a wavelength of light to be generated and/or detected in the microscopic aperture, the tip being filled with the transparent material. Accordingly, the near-field optical probe can illuminate near-field light to the sample by introducing light to the microscopic aperture and/or detect optical information in a microscopic region of a sample by the microscopic aperture.
Also, because the tip is filled with a transparent material, foreign matters will not intrude into the tip, enabling illumination and/or detection of near-field light with stable intensity. Furthermore, because the refractive index of the transparent member is greater than a refractive index of air, it is possible to increase the amount of near-field light passing through the microscopic aperture.
Also, the transparent material forming the tip and the transparent material forming the cantilever are structurally formed of a same transparent material. Accordingly, because there is no reflection between the tip and the cantilever, light incidence on the microscopic aperture and optical information detection from the microscopic aperture can be made with efficiency. Also, because the transparent material can be formed at one time in the manufacture process, the manufacturing method is facilitated. Furthermore, the transparent material is structurally silicon dioxide. Because silicon dioxide is one of the materials having high transmissivity in a visible portion of light, generation and detection of near-field light can be made with efficiency. Also, because silicon dioxide is a material generally used in the silicon process, it is favorable in control of form and mass producibility.
Also, the tip and the cantilever are structurally formed of transparent materials different in optical characteristic. Accordingly, if for example the cantilever is formed of silicon dioxide and the tip of diamond, the mechanical characteristic of the cantilever, such as resonant frequency, can be controlled with accuracy due to high formability of silicon dioxide and the wear resistance of the tip can be improved by the high wear resistance of diamond. Furthermore, in transparent materials, diamond is high in transmissivity and one of the materials extremely high in refractive index, making possible to increase the amount of near-field light transmitting through the microscopic aperture.
Also, the tip is structurally in a circular conical form. Accordingly, the microscopic aperture in outer shape is circular. By controlling the polarizing characteristic of incident light, near-field light having an arbitrary polarizing characteristic can be illuminated from the microscopic aperture.
Also, the tip structurally comprises a plurality of cones or pyramids different in angle of a side surface of the cone or pyramid. Accordingly, by reducing the tip end angle and increasing the taper angle to a middle of the tip, the near-field optical probe can be provided which satisfies at the same time high resolution of concave/convex images and optical images and generation efficiency of near-field light. Similarly, detection efficiency can be improved also in a collection mode for detecting optical information in a microscopic region of a sample by the microscopic aperture.
The cantilever structurally has a lens to focus incident light to the microscopic aperture and/or collimate light detected at the microscopic aperture. The lens is structurally a Fresnel lens formed on a side of the base of the cantilever or a refractive-index distribution type lens formed by controlling a refractive-index distribution in the cantilever. Accordingly, because the amount of light to be incident on the microscopic aperture can be increased, the intensity of near-field light illuminated from the microscopic aperture can be increased. Also, optical information of a sample can be detected efficiently by collimating the light detected by the microscopic aperture and guiding it to the detector by a focus lens.
Also, an end of the tip is structurally positioned nearly in a same plane as an end surface of the shade film. Accordingly, because the distance between the microscopic aperture and a sample can be made extremely short, the near-field light illuminated from and/or detected by the microscopic aperture can be converted into propagation light, improving the S/N ratio in the optical image. Also, improved is dissolving power of optical images.
Also, an end of the tip structurally protrudes greater than the end face of the shade film, an amount of protrusion thereof being equal to or smaller than a half of a wavelength of incident light on the microscopic aperture and/or light to be detected at the microscopic aperture. Accordingly, because the tip is small in radius of curvature, it is possible to improve resolution of concave/convex images and optical images for the scanning probe microscope. Furthermore, because the tip end and microscopic aperture center position are aligned, positional deviation is extremely small between a concave/convex image and an optical image.
Also, provided that a height of the tip is H, an inclination angle of the cantilever is xcex81, a spot diameter on the cantilever of incident light onto the tip and/or a spot diameter on the cantilever of light detected by the microscopic aperture and being incident on a detector is R1, and a distance of from a center of the tip to a free end of the cantilever is L1, L1 is structurally given satisfying
R1 less than L1 less than H/tan xcex81. 
Also, a tip of the cantilever structurally has a slant portion in such a form as spreading from the tip side to the base side. Also, a side surface of the cantilever structurally has a slant portion in such a form as spreading from the tip side to the base side. Furthermore, at a tip of the cantilever, a thin-sheet-formed connecting portion is structurally formed in a manner protruding toward the base, a thin-sheet-formed penthouse portion being formed extending parallel with the cantilever from the connecting portion.
With such a structure, because the tip at an end can access a sample and perfect light shade is possible excepting incident light and/or light detected by the microscopic aperture, it is possible to stably acquire convex/concave images and optical images.
Also, the near-field optical probe, because of capability of being fabricated using a silicon process, is high in mass producibility and good in shape reproducibility. Also, in a process for manufacturing a near-field optical probe, an outer shape forming process for the cantilever includes isotropic etching to form the slant portion. Also, in the method for manufacturing a near-field optical probe, a forming process for the connecting portion and the penthouse portion includes a process to form a step in a substrate and a process to form deposit the transparent material on the substrate. Accordingly, the near-field optical probe can be easily fabricated.
Also, in a near-field optical apparatus using a near-field optical probe, structure is made having an introducing/detecting optical system for introducing light to the microscopic aperture or detecting light from the microscopic aperture, detecting means for detecting a distance between the microscopic aperture and the sample, and a fine movement mechanism for finely moving the sample and/or the near-field optical probe, wherein the detecting means uses an optical lever method, a lens of the introducing/detecting optical system and a mirror of the detecting means being integrated together.
Also, in a near-field optical apparatus using a near-field optical probe, structure is made having an introducing/detecting optical system for introducing light to the microscopic aperture or detecting light from the microscopic aperture, detecting means for detecting a distance between the microscopic aperture and the sample, and a fine movement mechanism for finely moving the sample and/or the near-field optical probe, wherein the detecting means has a light source and an optical detector in a plane nearly vertical to the cantilever.
Also, the optical detector structurally detects reflection light upon the cantilever of light emitted from the light source.
Also, the optical detector structurally detects diffraction light upon the cantilever of light emitted from the light source.
Accordingly, because an introducing/detecting optical system having a great NA can be used free from interference between the detecting means and the introducing/detecting optical system, it is possible to emit near-field light great in light intensity from the microscopic aperture and, conversely, detect light from the microscopic aperture with efficiency.
Also, in a near-field optical apparatus using a near-field optical probe, structure is made having an introducing/detecting optical system for introducing light to the microscopic aperture or detecting light from the microscopic aperture, detecting means for detecting a distance between the microscopic aperture and the sample, and a fine movement mechanism for finely moving the sample and/or the near-field optical probe, wherein the detecting means detects interference at between an optical fiber arranged close to the cantilever and the cantilever.
Also, in a near-field optical apparatus using a near-field optical probe, structure is made having an introducing/detecting optical system for introducing light to the microscopic aperture or detecting light from the microscopic aperture, detecting means for detecting a distance between the microscopic aperture and the sample, and a fine movement mechanism for finely moving the sample and/or the near-field optical probe, wherein the detecting means is displacement detecting means of the cantilever provided on the near-field optical probe.
Accordingly, because an introducing/detecting optical system having a great NA can be used free from interference between the detecting means and the introducing/detecting optical system, it is possible to emit near-field light great in light intensity from the microscopic aperture and, conversely, detect light from the microscopic aperture with efficiency. Moreover, because the detecting means is small in size and lightweight, the near-field optical probe can be moved at high speed by the fine movement mechanism.
Also, in a near-field optical apparatus using a near-field optical probe, structure is made having an introducing/detecting optical system for introducing light to the microscopic aperture or detecting light from the microscopic aperture, detecting means for detecting a distance between the microscopic aperture and the sample, and a fine movement mechanism for finely moving the sample and/or the near-field optical probe, wherein the introducing/detecting optical system has an optical fiber provided at a tip a lens function.
Accordingly, because an introducing/detecting optical system having a great NA can be used free from interference between the detecting means and the introducing/detecting optical system, it is possible to emit near-field light great in light intensity from the microscopic aperture and, conversely, detect light from the microscopic aperture with efficiency. Moreover, because the introducing/detecting optical system is small in size and lightweight, the near-field probe can be moved at high speed by the fine movement mechanism.
Also, a near-field optical probe has, on the cantilever, a convex portion separate from the tip in position closer to the free end than a fixed end of the cantilever. Also, a near-field optical probe has the convex portion is formed on the cantilever on a side forming the tip in position closer to the fixed end than the tip.
Accordingly, the near-field optical probe resonant frequency can be lowered and optical-image S/N ratio can be improved. Also, because spring constant can be increased, it is possible to reduce the affection of air damping caused by the provision of the shade region, providing a near-field optical probe having stable operation characteristic.
Also, a near-field optical probe is made wherein the convex portion is formed on the cantilever on a side opposite to the side forming the tip.
Accordingly, a near-field optical probe can be obtained in which the convex portion will not contact a sample during observing a sample great in rise.
Also, in the process of manufacturing a near-field optical probe, included is a process for forming simultaneously a convex portion and a tip. A method for manufacturing a near-field optical probe includes a process to form a mold for forming the convex portion, a process of depositing a material to be formed into the convex portion in the mold, a process to planarize the material to be formed into the convex portion to bury the material to be formed into the convex portion in the mold, and a process to form the tip on the substrate buried with the material to be formed into the convex portion. Also, the process to planarize the material to be formed into the convex portion to bury the material to be formed into the convex portion in the mold is a polishing process.
Accordingly, it is possible to manufacture near-field optical probes low in resonant frequency and great in spring constant with high mass producibility, providing near-field optical probes inexpensive.